1. Field of the Invention
This invention relates to a method and device to measure fluid parameters of gas composition and temperature. It has particular application for the measurement of rapidly varying temperature in unsteady, compressible fluids, for example gas flowing in gas turbine engine ducts. Another example is the measurement of transient variation in the temperature of a gas which is subject to shock waves.
2. Discussion of Prior Art
Thermocouples are extensively used in many applications to provide accurate and low cost measurement of high temperatures. However their time response is poor due to thermal inertia effects and thus their use in temperature measurement of unsteady or fluctuating flowing fluids is restricted. Other devices, such as thin wire devices, have been used for temperature measurement but these are unsuitable for compressible or unsteady flows since the density, velocity, temperature and pressure of the flow (which are all required to obtain the temperature using these methods) vary independently with time. Optical pyrometry can only be used if optical access to the fluid is possible.
A device for temperature measurement in compressible flow using an aspirating probe has been described by Ng and Epstein in their paper "High Frequency Temperature and Pressure Probe for Unsteady Compressible flows" in Rev. Sci. Inst. Vol.54, No. 12, 1678-1683. This device comprises two constant temperature hot wires located upstream of a choked orifice. By operating the wires at different overheat ratios (probe temperature: flow recovery temperature), it is possible to temporarily resolve both pitot pressure and flow temperature variations in unsteady and fluctuating flow. The successful operation of the probe is dependant on establishing a steady flow in the aspirating channel leading up to and including a choked orifice both of which are parts of the probe itself. Calculation of flow establishment times and experimental measurements suggest that the upper frequency limit for this device is approximately 20 kHz.